A power system is configured with many power generators, loads, transmission and distribution devices, and controllers. In a normal state, small external disturbance may be applied to the power system due to load change, system switching, or the like. At the time of the occurrence of an accident such as ground fault or short-circuiting of the system, large external disturbance is applied to the power system. This external disturbance causes generation of power vibration including a plurality of vibration mode components.
In a case in which the vibration mode components are small or attenuated at a high speed, the system vibration after the occurrence of external disturbance is maintained in a sufficiently small range, and thus the power system is in a stable state. However, in a case in which the vibration becomes large or continues or disperses instead of being attenuated, consequently, the power generator stops, or power failure occurs in a wide range, and thus there is a possibility that the stable operation becomes impossible.
Together with an increase in the size of power systems, there is a tendency of an increase of a power supply capacity, an increase of the installation distance, uneven distribution, accordingly, an increase in the length of the transmission line, and an increase of the amount of electric currents flowing in lines. Since these changes cause the stability of the power system to be deteriorated, there is a concern of an expansion of a serial influence on large scale power dropout and power failure in a wide range caused by the step-out of the power generator at the time of system failure.
Therefore, an online system stability evaluation method which constantly monitors the stability of the system, and prevents and predicts the power generator step-out, and the like in advance becomes necessary.
Recently, a phasor measurement unit (PMU) which measures a phase angle of a bus voltage of the power system on almost a real-time basis by means of synchronous measurement using GPS has become widespread. It has become possible to understand a power vibration phenomenon on a real-time basis using an online measurement value such as the bus voltage of the system, or the like measured by the phasor measurement unit PMU.
From the above described background, a system state monitoring technology for an operator who operates the power system to monitor the stability of the system state has been devised. For example, in PTL 1, a power vibration mode observing apparatus, which accurately identifies an attenuation ratio playing an important role in steady state stability evaluation using a phase angle detecting apparatus, is disclosed.